Double doors, i.e. doors comprising two door leafs, are in some applications provided with a shoulder mounted on one of the door leafs such that the door leafs overlap. This implies that the door leafs have to be closed in a certain order in order to achieve proper closing of the door. If the shoulder is provided on the side of the door leafs facing the opening direction of the door, the door leaf without the shoulder should be closed first and then the door leaf with the shoulder should be closed. If the shoulder is provided on the side of the door leafs facing the closing direction of the door, the door leaf with the shoulder should be closed first and then the door leaf without the shoulder should be closed. The door leaf that should be closed first is denoted understriking door leaf or passive door leaf. The door leaf that should be closed last is denoted overstriking door leaf or active door leaf.
Proper closing of double doors is of course always important in order to achieve a full closing of the double door, but is in particular important in fire doors. Proper closing of doors is in particular important in fire doors where it is important to e.g. confine smoke and cut off oxygen supply. It is also advantageously to close doors properly to e.g. confine heat or cold or obstruct unauthorized access.
The proper closing order of the door leafs of a double door is in general achieved by blocking the closing motion of the active door leaf when the passive door leaf is open. The closing motion of the active door leaf is blocked by applying a breaking force on the active door leaf when the passive door leaf is open. When the passive door leaf is closed the breaking force is removed and thereby the active door leaf is allowed to close. The braking force is in generally applied on the active door leaf by applying a braking force on a motor shaft of a door operator controlling the opening of the active door leaf.